Selective Fluorescence Zinc Ion Sensing and Binding Behavior of 4

Jul 7, 2007 - DiVision, Indian Institute of Chemical Biology, JadaVpur, Kolkata-700 032, India. Received March 5, 2007. Zinc ion fluorescence sensing ...
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Inorg. Chem. 2007, 46, 6405−6412

Selective Fluorescence Zinc Ion Sensing and Binding Behavior of 4-Methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol: Biological Application Partha Roy,† Koushik Dhara,† Mario Manassero,‡ Jagnyeswar Ratha,§ and Pradyot Banerjee*,† Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Kolkata-700 032, India, Dipartimento di Chimica Strutturale e Stereochimica Inorganica dell’UniVersita´ di Milano, Via G. Venezian 21, 20133 Milano, Italy, and Cellular Biochemistry DiVision, Indian Institute of Chemical Biology, JadaVpur, Kolkata-700 032, India Received March 5, 2007

Zinc ion fluorescence sensing and the binding properties of 4-methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol (HL) have been investigated. It displays high selectivity for Zn2+ and can be used as zinc ion-selective luminescent probe for biological application under physiological conditions. The increase in emission in the presence of Zn2+ is accounted for by the formation of hexanuclear complex [Zn6(L)2(OH)2(CH3COO)8] characterized by X-ray crystallography. An approximately 6-fold Zn2+-selective chelation-enhanced fluorescence response in HEPES buffer (pH 7.4) is attributed due to the strong coordination of Zn(II) that would impose rigidity and hence decrease the nonradiative decay of the excited state. By incubation of cultured living cells (B16F10 mouse melanoma and A375 human melanoma) with HL, intracellular Zn2+ concentration could be monitored.

Introduction Search for reagents which can efficiently act as fluorescence sensors for zinc(II) has been an active area of research.1-7 The importance of zinc in the biological domain primarily lies in the fact that it is an essential trace element acting as a structural component of proteins and peptides. * To whom correspondence should be addressed. E-mail: icpb@ mahendra.iacs.res.in. Fax: (+91) 33 2473-2805. † Indian Association for the Cultivation of Science. ‡ Dipartimento di Chimica Strutturale e Stereochimica Inorganica dell’Universita´ di Milano. § Indian Institute of Chemical Biology. (1) (a) Liu, Y.; Zhang, N.; Chen, Y.; Wang, L.-H. Org. Lett. 2007, 9, 315. (b) Nolan, E. M.; Jaworski, J.; Racine, M. E.; Sheng, M.; Lippard, S. J. Inorg. Chem. 2006, 45, 9748. (c) Mikata, Y.; Wakamatsu, M.; Kawamura, A.; Yamanaka, N.; Yano, S.; Odani, A.; Morihiro, K.; Tamotsu, S. Inorg. Chem. 2006, 45 9262. (d) Nolan, E. M.; Jaworski, J.; Okamoto, K.-I.; Hayashi, Y.; Sheng, M.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 16812. (e) Nolan, E. M.; Burdette, S. C.; Harvey, J. H.; Hilderbrand, S. A.; Lippard, S. J. Inorg. Chem. 2004, 43, 2624. (f) Aoki, S.; Kaido, S.; Fujioka, H.; Kimura, E. Inorg. Chem. 2003, 42, 1023. (g) Koike, T.; Abe, T.; Takahashi, M.; Ohtani, K.; Kimura, E.; Shiro, M. J. Chem. Soc., Dalton Trans. 2002, 1764. (h) Burdette, S. C.; Walkup, G. K.; Spingler, B.; Tsien, R. Y.; Lippard, S. J. J. Am. Chem. Soc. 2001, 123, 7831. (i) Walkup, G. K.; Burdette, S. C.; Lippard, S. J.; Tsien, R. Y. J. Am. Chem. Soc. 2000, 112, 5644. (2) (a) Harrop, T. C.; Mascharak, P. K. Chemtracts 2001, 14, 442. (b) Kimura, E.; Aoki, S. BioMetals 2001, 14, 191. (c) Prasanna de Silva, A.; Nimal Gunaratne, H. Q.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515.

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Its role in the catalytic site of enzymes is also well-known.8 A topic of substantial current interest, however, stems out of the functionality of zinc in neurobiology.9 Recent studies have shown that a fraction of Zn2+ is found in free or chelatable form in some organs, e.g., brain,10 pancreas,11 and spermatozoa.12 A considerable amount of chelatable Zn2+ (3) (a) Czarnik, A. W. Acc. Chem. Res. 1994, 27, 302. (b) Akkaya, E. U.; Huston, M. E.; Czarnik, A. W. J. Am. Chem. Soc. 1990, 112, 3590. (c) Huston, M. E.; Engleman, C.; Czarnik, A. W. J. Am. Chem. Soc. 1990, 112, 7054. (4) (a) Kimura, E. S. Afr. J. Chem. 1997, 50, 240. (b) Kimura, E.; Koike, T. Chem. Soc. ReV. 1998, 27, 179. (c) Hendrickson, K. M.; Rodopoulos, T.; Pittet, P.-A.; Mahadevan, I.; Lincoln, S. F.; Ward, A. D.; Kurucsev, T.; Duckworth, P. A.; Forbes, I. J.; Zalewski, P. D.; Betts, H. J. Chem. Soc., Dalton Trans. 1997, 3879. (d) Koike, T.; Watanabe, T.; Aoki, S.; Kimura, E.; Shiro, M. J. Am. Chem. Soc. 1996, 118, 12696. (e) Coyle, P.; Zalewski, P. D.; Philcox, J. C.; Forbes, I. J.; Ward, A. D.; Lincoln, S. F.; Mahadevan, I.; Rofe, A. M. Biochem. J. 1994, 303, 781. (f) Zalewski, P. D.; Forbes, I. J.; Seamark, R. F.; Borlinghaus, R.; Betts, W. H.; Lincoln, S. F.; Ward, A. D. Chem. Biol. 1994, 1, 153. (g) Zalewski, P. D.; Forbes, I. J.; Betts, W. H. Biochem. J. 1993, 296, 403. (5) (a) Nasir, M. S.; Fahrni, C. J.; Suhy, D. A.; Kolodsick, K. J.; Singer, C. P.; O’Halloran, T. V. J. Biol. Inorg. Chem. 1999, 4, 775. (b) Fahrni, C. J.; O’Halloran, T. V. J. Am. Chem. Soc. 1999, 121, 11448. (c) Suhy, D. A.; Simon, K. D.; Linzer, D. I. H.; O’Halloran, T. V. J. Biol. Chem. 1999, 274, 9183. (6) (a) Gee, K. R.; Zhou, Z.-L.; Qian, W.-J.; Kennedy, R. J. Am. Chem. Soc. 2002, 124, 776. (b) Hirano, T.; Kikuchi, K.; Urano, Y.; Higuchi, T.; Nagano, T. Angew. Chem., Int. Ed. 2000, 39, 1052. (c) Godwin, H. A.; Berg, J. M. J. Am. Chem. Soc. 1996, 118, 6514.

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Roy et al. sequestered in the vesicles of presynaptic neurons is released from brain when the neurons become active.13 However, the effective role of Zn2+ is poorly understood with neuronal disorders.14 An upper limit of estimate ranging to the low millimolar15 concentration of Zn2+ within the neuronal vesicles has been made, and the same in the synapse falls at 10-30 µM.14,15 Neuronal death can occur owing to uncontrolled Zn2+ released from the mossy fiber terminals after traumatic brain injury, stock, or seizure.16 It is also believed that the Zn2+ homeostasis may have some bearing to the pathology of Alzheimer’s disease and other neurological problems.14,17 The lack of appropriate detecting tools for over a quite large concentration range causes a severe hindrance for further investigations of this spectroscopically inaccessible metal ion. For imaging biological Zn2+, the use of several fluorescent zinc chemisensors is in vogue.2b,18 The common probes are aryl sulfonamide derivatives of 8-aminoquinoline, such as 6-methoxy-(8-p-toluenesulfonamido)quinoline (TSQ)19 and Zinquin,4f as well as Zinbo-5,20 the Zinpyr (ZP) family,1h,1i,21-23 and the ZnAF molecules.24 The water solubility of TSQ poses some problem which has been averted by the introduction of carboxylic acid groups or ester groups to extend the 6-methoxy group4f,25 and replacement of the methyl group (7) (a) Pearce, D. A.; Jotterand, N.; Carrico, I. S.; Imperiali, B. J. Am. Chem. Soc. 2001, 123, 5160. (b) Prodi, L.; Montali, M.; Bradshaw, J. D.; Izatt, R. M.; Savage, P. B. J. Inclusion Phenom. Macrocycl. Chem. 2001, 41, 123. (c) Reany, O.; Gunnlaugsson, T.; Parker, D. J. Chem. Soc., Chem. Commun. 2000, 473. (d) Walkup, G. K.; Imperiali, B. J. Org. Chem. 1998, 63, 6727. (e) Walkup, G. K.; Imperiali, B. J. Am. Chem. Soc. 1997, 119, 3443. (f) Fabbrizzi, L.; Licchelli, M.; Pallavicin, P.; Perotti, A.; Taglietti, A.; Sacchi, D. Chem.sEur. J. 1996, 2, 75. (g) Walkup, G. K.; Imperiali, B. J. Am. Chem. Soc. 1996, 118, 3053. (8) (a) Vallee, B. L.; Falchuk, K. H. Physiol. ReV. 1993, 73, 79. (b) Berg, J. M.; Shi, Y. G. Science 1996, 271, 1081. (9) (a) Vallee, B. L.; Falchuk, K. H. Physiol. ReV. 1993, 73, 79-118. (b) Frederickson, C. J.; Moncrieff, D. W. Biol. Signals 1994, 3, 127. (c) Takeda, A. BioMetals 2001, 14, 343. (10) (a) Frederickson, C. J. Int. ReV. Neurobiol. 1989, 31, 145. (b) Frederickson, C. J.; Bush, A. I. Biometals 2001, 14, 353. (c) Li, Y.; Hough, C.; Sarvey, J. Sci. STKE 2003, 182, 19. (11) (a) Zalewski, P. D.; Millard, S. H.; Forbes, I. J.; Kapaniris, O.; Slavotinek, A.; Betts, W. H.; Ward, A. D.; Lincoln, S. F.; Mahadevan, I. J. Histochem. Cytochem. 1994, 42, 877. (b) Qian, W. J.; Gee, K. R.; Kennedy, R. T. Anal. Chem. 2003, 75, 3468. (12) Zalewski, P. D.; Jian, X.; Soon, L. L. L.; Breed, W. G.; Seamark, R. F.; Lincoln, S. F.; Ward, A. D.; Sun, F. Z. Reprod., Fertil. DeV. 1996, 8, 1097. (13) (a) Assaf, S. Y.; Chung, S. H. Nature 1984, 308, 734. (b) Howell, G. A.; Welch, M. G.; Frederickson, C. J. Nature 1984, 308, 736. (14) Bush, A. I. Curr. Opin. Chem. Biol. 2000, 4, 184. (15) Frederickson, C. J.; Bush, A. I. Biometals 2001, 14, 353. (16) (a) Suh, S. W.; Chen, J. W.; Motamedi, M.; Bell, B.; Listiak, K.; Pons, N. F.; Danscher, G.; Frederickson, C. J. Brain Res. 2000, 852, 268. (b) Choi, D. W.; Koh, J. Y. Annu. ReV. Neurosci. 1998, 21, 347. (17) Cuajungco, M. P.; Lees, G. J. Neurobiol. Dis. 1997, 4, 137. (18) Jiang, P.; Guo, Z. Coord. Chem. Rev. 2004, 248, 205. (19) Frederickson, C. J.; Kasarskis, E. J.; Ringo, D.; Frederickson, R. E. J. Neurosci. Methods 1987, 20, 91. (20) Taki, M.; Wolford, J. L.; O’ Halloran, T. V. J. Am. Chem. Soc. 2004, 126, 712. (21) Burdette, S. C.; Frederickson, C. J.; Bu, W.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125, 1778. (22) Chang, C. J.; Nolan, E. M.; Jaworski, J.; Okamoto, K.-I.; Hayashi, Y.; Sheng, M.; Lippard, S. J. Inorg. Chem. 2004, 43, 6774. (23) Chang, C. J.; Nolan, E. M.; Jaworski, J.; Burdette, S. C.; Sheng, M.; Lippard, S. J. Chem. Biol. 2004, 11, 203. (24) (a) Hirano, T.; Kikuchi, K.; Urano, Y.; Nagano, T. J. Am. Chem. Soc. 2002, 124, 6555 (b) Hirano, T.; Kikuchi, K.; Urano, Y.; Higuchi, T.; Nagano, T. J. Am. Chem. Soc. 2000, 122, 12399.

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on the benzene ring with a carboxylic acid group substituent.26 Difficulty arising from the use of such probes lies in their sparing solubility in neutral aqueous solution. Apart from these type of sensors, use of synthetic peptides for signaling zinc is also made which act by the way of fluorescence enhancement.6c,27 The aryl sulfonamide sensors can in general form 1:1 and 1:2 Zn2+/ligand complexes in such a way that it is possible for a protein-bound zinc ion to activate the change in the dye fluorescence.18 There are other types of sensors which bind zinc to form either exclusively 1:1 complexes20,21,24,28 or a mixture of 1:1 and 1:2 species (abbreviated as ZP1 and ZP2).1h,i These can impart significant fluorescence changes only when the first Zn2+ ion is bound. Although a variety of zinc sensors are reported, there remains much to be done in this aspect. The most powerful approach seems to design small and simple synthetic molecules that combine a Zn2+ binding unit with a fluorophore. Significant change in either fluorescence intensity or wavelength on Zn2+ coordination occurs making it a suitable analytical tool. Herein we explore the selective Zn2+ sensor property of a simple and small molecule, 4-methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol (HL), prepared by Schiff base condensation of 4-methyl-2,6-diformylphenol and phenylmethylamine in HEPES buffer (pH 7.4) at 25 °C. The investigations on the luminescent property in the working buffer show that the emission intensity of HL increases significantly upon addition of various concentrations of Zn2+, while the introduction of other transition metal ions and biologically relevant metal ions (Ca2+, Mg2+, Na+, and K+) causes the intensity to be either unchanged or weakened. The system shows selective chelation enhanced fluorescence (CHEF) in the presence of Zn(II) ion. The enhancement of fluorescence is attributed due to the strong binding of Zn(II) evident from a large binding constant value (1.5 × 104 M-1) that would impose rigidity and hence decrease the nonradiative decay of the excited state. The case implies that HL could monitor or recognize Zn2+ and be considered as a selective luminescent probe. Reaction of zinc(II) acetate and HL affords the hexanuclear neutral complex [Zn6(L)2(OH)2(CH3COO)8], (1) characterized by elemental analysis and single-crystal X-ray structural determination. HL exhibits satisfactory Zn2+ sensing abilities in the physiological pH range. We have examined the application of the sensor (HL) to cultured living cells (B16F10 mouse melanoma and A375 human melanoma) by fluorescence microscopy. Experimental Section Materials and Physical Methods. High-purity HEPES and zinc acetate dihydrate were purchased from Sigma Aldrich and were used as received. All other reagents were of analytical grade. Buffer was prepared using deionized and sonicated triple distilled water. (25) Mahadevan, I. B.; Kimber, M. C.; Lincoln, S. F.; Tiekink, E. R. T.; Ward, A. D.; Betts, W. H.; Forbes, I. J.; Zalewski, P. D. Aust. J. Chem. 1996, 49, 561. (26) Budde, T.; Minta, A.; White, J. A.; Kay, A. R. Neuroscience 1997, 79, 347. (27) Shults, M. D.; Pearce, D. A.; Imperiali, B. J. Am. Chem. Soc. 2003, 125, 10591. (28) Henary, M. M.; Fahrni, C. J. J. Phys. Chem. A 2002, 106, 5210.

SelectiWe Fluorescence Zinc Ion Sensing Scheme 1

Solvents used for spectroscopic studies were purified and dried by standard procedures before use.29 FT-IR spectra were obtained on a Nicolet MAGNA-IR 750 spectrometer with samples prepared as KBr pellets. Elemental analysis was carried out in a 2400 Series-II CHN analyzer, Perkin-Elmer, Norwalk, CT. Absorption and luminescence spectra were studied on a Shimadzu UV2100 UVvis recording spectrophotometer and a Perkin-Elmer LS 55 Luminescence Spectrometer, respectively. The ESI-MS was recorded on Qtof Micro YA263 mass spectrometer. Fluorescence life times were determined from time-resolved intensity decay by the method of time-correlated single-photon counting using a picosecond diode laser at 403 nm as light source. Imaging System. The imaging system was comprised of an inverted fluorescence microscope (IX-70, Olympus, Tokyo, Japan), digital compact camera (Olympus, model C-5060), and an image processor (CAMEDIA Master, Olympus). The microscope was equipped with a halogen lamp, a mercury burner, and objective lens [×40 (B16F10 and A375)]. Preparation of Cells. A375 human melanoma and B16F10 mouse melanoma cells procured from National Center for Cell Science, Pune, India, were cultured in Dulbecco’s modified Eagle’s medium (DMEM, Gibco BRL) supplemented with 10% FBS (Gibco BRL) and 1% antibiotic mixture containing penicillin, streptomycin, and neomycin (PSN, Gibco BRL) at 37 °C in a humidified incubator with 5% CO2. For experimental study, cells were grown to 8090% confluence, harvested with 0.025% trypsin (Gibco BRL) and 0.52 mM EDTA (Gibco BRL) in PBS (phosphate-buffered saline, Sigma Diagnostics), plated in a six-well culture plate (NUNC, Roskilde, Germany) at a density of 5 × 105 cells/mL, and allowed to reequilibrate for 24 h before loading. Then the cells were rinsed with PBS and incubated with PBS containing 20 µM of HL for 30 min at 37 °C. The cells were then washed with PBS thrice and mounted on a microscope stage. Synthesis of 4-Methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol (HL). The compound was prepared by a slight modification of the procedure described previously.30 4-Methyl-2,6-diformylphenol was synthesized starting from p-cresol by following a published procedure.31 To a solution of 4-methyl-2,6-diformylphenol (0.328 g, 2 mmol) in 25 mL of acetonitrile was added phenylmethylamine (0.428 g, 4 mmol) in 10 mL of acetonitrile. The reaction mixture was refluxed for 1 h. The solution was filtered, concentrated on a rotaevaporator to dryness, and kept overnight at 4 °C. The resulting Schiff base, HL (4-methyl-2,6-bis(((phenylmethyl)imino)methyl)(29) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; Pergamon Press: Oxford, U.K., 1980. (30) Grzybowski, J. J.; Urbach, F. L. Inorg. Chem. 1980, 19, 2604. (31) Gagne, R. R.; Spiro, C. L.; Smith, T. J.; Hamann, C. A.; Thies, W. R.; Schiemke, A. K. J. Am. Chem. Soc. 1981, 103, 4073.

phenol) (Scheme 1) is solid (yield ) 0.58 g, 85%). Data for HL are as follow. Anal. Calcd for C23H22N2O: C, 80.67; H, 6.48; N, 8.18. Found: C, 80.58; H, 6.42; N, 8.10. FT-IR (KBr phase) (ν/ cm-1): 1635, 1600. 1H NMR (300 MHz, CDCl3; δ): 2.30 (s, 3H, Ar-CH3), 4.83 (s, 4H, CH), 7.26-7.40 (m, 12H, Ar-CH), 8.67 (s, 2H, HCdN). HRMS [(M + H+)/z, (%)]: simulated for C23H23N2O, 343.1797 (100); found, 343.0690 (100). Synthesis of [Zn6(L)2(OH)2(CH3COO)8] (1). A 10 mL acetonitrile solution of zinc(II) acetate dihydrate (0.131 g, 0.6 mmol) was added slowly to a magnetically stirred 10 mL acetonitrile solution of the ligand (HL) (0.068 g, 0.2 mmol). The mixture was stirred in air for 30 min whereby a yellow solution was formed. It was filtered and kept in air. Pale yellow prismatic single crystals of 1 suitable for X-ray crystallography were obtained on slow evaporation of the filtrate at ambient temperature within 3 days (yield: 80%). Anal. Calcd for C62H68N4O20Zn6: C, 47.09; H, 4.33; N, 3.54. Found: C, 47.11; H, 4.30; N, 3.48. FT-IR (KBr phase) (cm-1): 3433 br, 3215 br, 1647 vs, 1556 vs, 1452 m, 1323 m, 1238 w, 1113 vs, 625 s, 489 w (br, broad; w, weak; m, medium; s, strong; vs, very strong). X-ray Data Collection and Structure Determination. Crystal data of 1 are summarized in Table 1. The diffraction experiment was carried out on a Bruker SMART CCD area-detector diffractometer at 296 K. No crystal decay was observed, so that no timedecay correction was needed. The collected frames were processed with the software SAINT,32 and an empirical absorption correction was applied (SADABS)33 to the collected reflections. The calculations were performed using the Personal Structure Determination Package34 and the physical constants tabulated therein.35 The structure was solved by direct methods (SHELXS)36 and refined by full-matrix least squares using all reflections and minimizing the function Σw(Fo2 - kFc2)2 (refinement on F2). All the nonhydrogen atoms were refined with anisotropic thermal factors. The 15 hydrogen atoms of the 5 CH3 groups were refined with an isotropic thermal parameter. All the other hydrogen atoms were placed in their ideal positions (C-H or O-H ) 0.97 Å), with the thermal parameter U 1.10 times that of the atom to which they are attached, and not refined. In the final Fourier map the maximum residual was 1.35(20) e Å-3 at 0.87 Å from Zn1. CCDC 638646 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the (32) SAINT Reference Manual; Siemens Energy and Automation: Madison, WI, 1994-1996. (33) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program; University of Gottingen: Gottingen, Germany, 1997. (34) Frenz, B. A. Comput. Phys. 1988, 2, 42. (35) Crystallographic Computing 5; Oxford University Press: Oxford, U.K., 1991; Chapter 11, p 126.

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Roy et al. Table 1. Crystallographic Data for 1 formula M color cryst system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z F(000) Dc/g cm-3 T/K λ(Mo KR)/Å cryst dimens/mm µ(Mo KR)/cm-1 min and max transm factors scan mode frame width/deg time frame-1/s no. of frames detector-sample dist/cm θ-range/deg reciprocal space explored no. of reflcns (tot., indpndt) Rint final R2 and R2w indicesa (F2, all reflcns) conventional R1 index [I > 2σ(I)] reflcns with I > 2σ(I) no. of variables goodness of fitb

C62H68N4O20Zn6 1581.47 pale yellow triclinic P1h 10.740(1) 12.927(1) 13.281(1) 115.06(1) 96.34(1) 101.98(1) 1592.5(2) 1 808 1.649 296 0.710 73 0.21 × 0.35 × 0.36 23.56 0.853-1.000 w 0.30 20 2450 6.00 3-27 full sphere 24 368, 8042 0.0185 0.055, 0.084 0.034 5818 475 1.065

2 2 2 2 2 1/2 a R ) [Σ(|F 2 - kF 2|/ΣF 2], R 2 o c o 2w ) [Σw/(Fo - kFc ) /Σw(Fo ) ] . [Σw(Fo2 - kFc2)2/(No - Nv)]1/2, where w ) 4Fo2/σ(Fo2)2, σ(Fo2) ) [ σ2(Fo2) + (pFo2)2]1/2, No is the number of observations, Nv is the number of variables, and p ) 0.04.

b

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

Results and Discussion Fluorescence Property. The emission spectrum of the sensor (HL) under physiological condition (100 mM HEPES buffer at pH 7.4) excited at 400 nm exhibits the emission maximum at 530 nm at 25 °C with a quantum yield of only 0.040. We measured the emission intensities of the sensor molecule in the presence of various concentrations of Zn2+ (0-1.0 mM concentration). The sensor showed changes in fluorescence intensity and a new peak appeared at 484 nm, retaining 530 nm as a shoulder as suggested by unsymmetrical shape of the emission spectrum (Figure 1), and the fluorescence quantum yield (Φ ) 0.250) increased more than 6-fold. The fluorophore and the guest binding units were covalently integrated to modulate the photophysical responses of the ligand, HL, by introducing a possible excited-state electron-transfer process between the two. The system shows selective chelation enhanced fluorescence (CHEF) in the presence of Zn2+ ion. The experiment reveals that there is a significant increase in fluorescence intensity for 1. The new peak of emission band (484 nm) by the complex 1 as compared to that at 530 nm of the free ligand is quite (36) Sheldrick, G. M. SHELXS 86. Program for the solution of crystal structures; University of Gottingen: Gottingen, Germany, 1985.

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Figure 1. Emission spectra of 50 µM of HL in the presence of 0, 10, 30, 50, 75, 100, 150, 200, 400, 600, 800, and 1000 µM of free Zn2+ ions in 100 mM HEPES buffer at pH 7.4 at room temperature (excitation 400 nm, emission 530 nm). Inset: Fluorescence enhancement versus concentration of Zn2+.

unexpected (Figure 1), as one would expect the emission band of the free ligand to be at higher energy related to that of the complex because of its relatively high-energy excitation band. This anomaly can be explained by the fact that excited states resulting from complexes of Zn2+ are typically ligand-centered (LC) in nature owing to the inability of the d10 metal center to participate in low-energy charge transfer for metal-centered transitions.37 Therefore, the emission from [Zn6(L)2(OH)2(CH3COO)8] (1) is believed to arise from an excited-state centered on HL which contains a phenolic proton (Scheme 1). It is well-established that the emissive organic phenol, 4-methyl-2,6-diformylphenol, undergoes a blue shift of its own band (from 530 to 445 nm) upon treatment with a base B (e.g., piperidine or triethylamine), and this observation is believed to support the existence of an equilibrium between undissociated phenol and a protontransferred species of phenol with the formation of phenoxide anion and BH+ cation.38 The same analogy can be extended to the present system because the phenolic proton of the ligand is deprotonated upon complexation with Zn2+, and this proton should be transferred to the acetate anion of the precursor zinc(II) acetate dihydrate in the solution. We performed the emission experiment on the free ligand in the presence of an added base, e.g., triethylamine, in aqueous solution and found that, on addition of triethylamine, a new band appeared at around 458 nm. This indicates the existence of an equilibrium between an undissociated HL and a protontransferred species of HL (see Supporting Information, Figure S1). The electrospray mass spectrum of the complex 1 was obtained in aqueous solution. The spectrum shows peaks at m/z 1617.76 and 1322.73 that can be assigned to the [Zn6(L)2(OH)2(CH3COO)8]‚2H2O and [Zn4(L)2(OH)2(CH3COO)4(H2O)6] species, respectively. The isotopic distributions for the above molecular species obtained experimentally (see (37) (a) Dollberg, C. L.; Turro, C. Inorg. Chem. 2001, 40, 2484. (b) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Fackler, J. P., Ed.; Modern Inorganic Chemistry Series; Plenum Press: New York, 1994; pp 56-57. (38) (a) Mukherjee, S. Indian J. Chem. 1987, 26A, 1002. (b) Das, R.; Mitra, S.; Mukherjee, S. Bull. Chem. Soc. Jpn. 1993, 66, 2492.

SelectiWe Fluorescence Zinc Ion Sensing

Figure 2. Binding constant, K ((15%), value determined from the slope of the plot as 1.5 × 104 M-1.

Supporting Information, Figure S2) correspond to the simulated patterns. In this spectrum a peak at m/z 1617.76 assigned to [Zn6(L)2(OH)2(CH3COO)8]‚2H2O and a peak at m/z 1322.73 that corresponds to the [Zn4(L)2(OH)2(CH3COO)4(H2O)6] species corroborate the presence of the complex entity in solutions. Job’s plot analysis revealed maximum emission at 1:3 ratio (HL:Zn2+) (see Supporting Information, Figure S3). These data suggested that the complex species in solution should form 1:3 complex with Zn2+ which is clearly evident from the crystal structure of Zn2+-ligand complex 1 (see later). To observe the binding interaction with Zn2+ in buffer medium, the binding constant value has been determined from the emission intensity data following the modified Benesi-Hildebrand equation:39a,b 1/∆F ) 1/∆Fmax + (1/K[C])(1/∆Fmax). Here ∆F ) Fx - F0 and ∆Fmax ) F∞ - F0, where F0, Fx, and F∞ are the emission intensities of HL considered in the absence of Zn2+, at an intermediate Zn2+ concentration, and at a concentration of complete interaction, respectively, and where K is the binding constant and [C] the Zn2+ concentration. From the plot of (F∞ - F0)/(Fx - F0) against [C]-1 for HL (Figure 2), the value of K ((15%) extracted from the slope is 1.5 × 104 M-1. We also studied the effect of Zn2+ addition on the fluorescence decay behavior of HL. The fluorescence lifetime (τ) of HL in the working buffer medium is 2.09 ns. The results of fluorescence lifetime measurements (see Supporting Information, Figure S4) indicate that changes in lifetime would be associated with any number of possible mechanisms for fluorescence enhancement and not only PET (photoinduced electron transfer). The time-resolved spectrum appears to display at least two components. Since the mechanism of fluorescence enhancement is a change in rigidity upon binding of zinc ion, a different mechanism other than PET may be of significance. According to the equations39c τ-1 ) kr + knr and kr ) Φf/τ, the radiative rate constant kr and total nonradiative rate constant knr of HL and HL binding with Zn2+ were calculated and listed in Table S1. The data suggest that knr has just slightly changed but the factor that induces fluorescent enhancement is mainly ascribed to the increase of kr. (39) (a) Mallick, A.; Chahattopadhyay, N. Photochem. Photobiol. 2005, 81, 419. (b) Benesi, H. A.; Hildebrand, J. H.; J. Am. Chem. Soc. 1949, 71, 2703. (c) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings Publishing Co., Inc.: Menlo Park, CA, 1978.

Figure 3. UV-vis spectral changes of HL upon addition of Zn2+ in 100 mM HEPES buffer at pH 7.4 at room temperature ([HL] ) 50 µM, [Zn2+] ) 0, 10, 25, 50, 75, 100, 125, 150 µM).

Upon gradual addition of Zn2+ to a buffer solution of HL, the fluorescence intensity of the 530 nm band is meagerly changed along with the emergence of a new emission bend centered at 484 nm. It allows the ratiometric signaling when the ratio of the fluorescence output at two different wavelengths is plotted as a function of the concentration of Zn2+ (see Supporting Information, Figure S5). As can be seen, I484/I530 increases with the concentration of Zn2+ ions in the range studied. Absorption Study. The mode of coordination of HL with Zn2+ was investigated by spectrophotometric titration at 25 °C in 100 mM HEPES buffer (pH 7.4). Figure 3 illustrates a typical UV-vis titration curve of HL with added Zn2+. As can be seen from Figure 3, the absorption intensity of HL at 234 nm gradually increases as the concentration of Zn2+ increases stepwise. Moreover, a new absorption peak appears at 418 nm in the UV-vis spectrum of the HLZn2+ system, and its intensity also gradually increases with the addition of Zn2+. This absorption peak is likely to be due to the coordination of HL with Zn2+.40 Metal Ion Competition Studies. To enable further understanding of this phenomenon, we have examined the effects of various metal ion additives on the spectral features of HL. Selectivity assays (Figure 4) were performed under physiological condition (100 mM HEPES buffer at pH 7.4) excited at 400 nm. Figure 4 illustrates the fluorescence intensities of HL (50 × 10-6 M) in presence of various metal ions. It is observed that, among the metal ions studied, only Zn2+ significantly modulates the fluorescence spectra of HL. Other cations which exist at high concentrations in living cells, e.g., Ca2+, Mg2+, Na+, and K+, do not enhance the fluorescence intensity even at a high concentration (150 × 10-6 M) as shown in Figure 4. These results are presumably due to the poor complexation of alkali metals or alkaline earth metals with the chelator (HL), on the basis of the absence of any obvious change of UV-visible spectrum upon addition of these cations. This also does not interfere with the Zn2+-induced fluorescence enhancement. Emission spectrum of HL (50 × 10-6 M) was recorded upon addition (40) Prabhakar, M.; Zacharias, P. S.; Das, S. K. Inorg. Chem. 2005, 44, 2585.

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Figure 6. Fluorescence intensities of HL in the absence (open circles) and in the presence of Zn2+ (closed circles) at various pH values at room temperature (150 µM Zn2+ added to 50 µM of HL). Table 2. Selected Bond Lengths (Å) and Bond Angles (deg)a

Figure 4. Relative fluorescence intensity change profile of HL (50 µM) in the presence of various cations in 100 mM HEPES buffer at pH 7.4 at room temperature (excitation 400 nm).

Zn1-O1 Zn1-O2 Zn1-O4a Zn1-O10 Zn1-N1 Zn2-O1 Zn2-O2 Zn2-O3 O1-Zn1-O2 O1-Zn1-O10 O2-Zn1-N1 O10-Zn1-N1 O1-Zn1-O4a O4a-Zn1-O10 O1-Zn2-O2 O1-Zn2-O3 O2-Zn2-N2 O3-Zn2-N3 a

Figure 5. ORTEP drawing and atom numbering scheme for [Zn6(L)2(OH)2(CH3COO)8] (1). H atoms were omitted for clarity. The atoms are represented by 50% probability thermal ellipsoids.

of 3 equiv (150 × 10-6 M) of various heavy metal ions M (M ) Mn2+, Fe3+, Cr3+, Co2+, Ni2+, Cu2+, Cd2+). Among first-row transition metal cations, Cr, Mn, Fe, Co, Ni, and Cu quench the emission, owing to an electron or energy transfer between the metal cation and fluorophore known as the fluorescence quenching mechanism.41 In the presence of 3 equiv of Cu2+, Fe3+, or Mn2+ together with Zn2+ (50 × 10-6 M), HL has almost no effect on the emission intensities. Cd2+ induces a slight enhancement of the emission intensity. However, Cd2+ is rarely present in biological systems and would not cause any problem in biological applications. The enhancement of fluorescence is attributed to the introduction of Zn2+ and the strong complexation occurs with HL evident from large binding constant value. The coordination of Zn(II) would impose rigidity and hence decrease the nonradia(41) Sarkar, M.; Banthia, S.; Samanta, A. Tetrahedron Lett. 2006, 47, 7575.

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2.024(1) 2.105(1) 2.019(1) 2.044(1) 2.045(2) 2.006(1) 2.077(1) 2.116(1) 78.00(5) 94.72(7) 86.53(5) 91.42(7) 96.42(5) 104.85(8) 79.06(5) 88.65(5) 90.66(7) 99.22(6)

Zn2-O5 Zn2-O7a Zn2-N2 Zn3-O1 Zn3-O6 Zn3-O8 Zn3-O9 O1-Zn2-O5 O2-Zn2-O5 O1-Zn2-O7a O2-Zn2-O7a O1-Zn3-O6 O1-Zn3-O8 O1-Zn3-O9 O6-Zn3-O8 O6-Zn3-O9 O8-Zn3-O9

2.118(2) 2.439(2) 2.020(1) 1.949(1) 1.956(2) 1.935(2) 1.958(2) 96.34(5) 95.26(7) 89.17(5) 86.28(6) 106.28(7) 121.71(6) 108.24(7) 103.27(8) 105.43(9) 110.58(8)

Symmetry codes: (a) 1 - x, 1 - y, 1 - z.

tive decay of the excited state. Thus, Zn2+ can be distinguished from these heavy metal ions under physiological conditions. Crystal Structure of 1. Complex 1 crystallizes in the triclinic system, space group P1h (No. 2) from acetonitrile solvent. A perspective view of [Zn6(L)2(OH)2(CH3COO)8] (1) with the atom-numbering scheme is shown in Figure 5. Selected bond lengths and bond angles are given in Table 2. This is a discrete hexanuclear complex. The asymmetric unit of complex 1 consists of three Zn2+, one binucleating tridentate ligand (4-methyl-2,6-bis(((phenylmethyl)imino)methyl)phenolate), one µ3-hydroxide, and four acetate ions. Atom Zn1 is coordinated by a µ2-phenoxo oxygen atom (O2) and N1 atom of the ligand HL, a µ3-hydroxo oxygen atom (O1), and two µ2-acetato oxygen atoms {O4a [(a) 1 - x, 1 - y, 1 - z) and O10}. On the other hand, Zn2 is connected by the µ2-phenoxo oxygen atom (O2) and N2 atom of the ligand, the same µ3-hydroxo oxygen atom, and three µ2acetato oxygen atoms (O3, O5, and O7a, “a” as above). As a consequence, Zn1 and Zn2 display coordination numbers 5 and 6, respectively, while Zn3 has the coordination number 4 and is linked by the same µ3-hydroxo oxygen atom and three µ2-acetato oxygen atoms (O6, O8, and O9). It is quite evident from the crystal structure that the three Zn2+ ions in the asymmetric unit have three different environments in their coordination spheres. Hexacoordinated Zn2 adopts a distorted

SelectiWe Fluorescence Zinc Ion Sensing

Figure 7. Phase contrast and fluorescence response of HL induced by intracellular Zn2+. B16F10 and A375 cells incubated with 20 µM HL for 45 min at room temperature were washed with PBS, and fluorescence excited at 385-420 nm was measured at 30 s intervals. Cells were exposed to pyrithione (pyr, 100 µM) in the presence of sequentially increased concentration of added extracellular Zn2+ as indicated by (a) (closed circles for B16F10 and open circles for A375). Return of intracellular Zn2+ to the resting level was achieved by addition of TPEN (200 µM, indicated by the arrow). (b) Phase contrast image. (c-f) Fluorescence images incubated with HL (c, 2 min), with pyr and 50 µM Zn2+ (d, 7 min), with 150 µM Zn2+ (e, 17 min), and with 200 µM TPEN (f, 23 min). Top row: B16F10. Bottom row: A375.

octahedral geometry with cis angles ranging from 79.06(5) to 99.22(6)°. The pentacoordinated zinc atom (Zn1) is in a distorted square-pyramidal geometry, and the zinc ion is displaced by 0.502(1) Å from the best plane formed by atoms N1, O1, O2, and O10. The axial Zn1-O4a [(a) 1 - x, 1 y, 1 - z) distance is 2.019(1).42 Zn3 is in a distorted tetrahedral geometry with the bond angles ranging from 103.27(8) to 110.58(8)°. Effect of pH. In addition to metal ion selectivity, we measured the fluorescence intensity of HL at various pH values in the presence and absence of Zn2+. The fluorescence intensity of the HL decreases under acidic condition. This property arises from protonation of the phenolic hydroxyl group of the ligand, whose pKa value has been determined to be 5.19.43 Interestingly, HL showed good fluorescence sensing ability to Zn2+ over a wide range of pH values. As shown in Figure 6, HL shows no appreciable sensing ability to Zn2+ at a pH below 6.3, which may be due to the competition of H+ below pH 6.3, but exhibits satisfactory Zn2+ sensing abilities when the pH is increased to the range 6.3-10. At pH ca. 7.4, the FHL/Zn(II)/FHL reached its maximum value, indicating that HL possessed the highest sensing ability in an environment similar to serum (pH ca. 7.3). The pH sensitivity of the sensor is reflected by the drop of emission intensity by ca. 40% on going from pH 7.5 to (42) Yamami, M.; Furutachi, H.; Yokoyama, T.; O h kawa, H. Inorg. Chem. 1998, 37, 6832. (43) Albert, A.; Serjeant, E. P. Determination of Ionization Constants. A Laboratary Manual, 3rd ed.; Chapman and Hall: London, 1984.

7.0. As the local pH can drop below 6.5 in many cell compartments and biofluids, the utility of this probe is restricted. Cell Studies. We examined the application of HL to cultured living cells B16F10 mouse melanoma and A375 human melanoma by fluorescence microscopy (Figure 7). Since HL can be excited with a relatively long excitation wavelength, this compound is suitable for cellular application in comparison to the previously reported Zn2+-selective luminescent sensors.44 After incubation with 20 µM sensor, cells show weak fluorescence suggesting that the sensor molecule can be used intracellularly. We further examined whether the difference of affinity is reflected in the fluorescence microscopic imaging of intracellular Zn2+ or not. The intracellular concentration of Zn2+ was controlled with a Zn2+ ionophore, pyrithione (2-mercaptopyridine N-oxide), which brings the extracellular Zn2+ into the cytoplasm. We added various concentrations of Zn2+ with pyrithione, and the intracellular Zn2+ concentration was measured by fluorescence microscopy. With the increase of extracellular Zn2+ concentration, the fluorescence intensity of the compound increases. Figure 7 shows the fluorescence response of the sensor molecule. The fluorescence was measured at 30 s intervals for the different concentrations of zinc(II) acetate. The fluorescence intensity was saturated with 150 µM Zn(OAc)2. Subsequent addition of the cell-permeable metal (44) Hanaoka, K.; Kikuchi, K.; Kojima, H.; Urano, Y.; Nagano, T. J. Am. Chem. Soc. 2004, 126, 12470.

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Roy et al. chelator N,N,N′,N′-tetrakis(2-pyridyl)ethylenediamine (TPEN) quenches the emission intensity consistent with the intracellular turn-on as a consequence of reversible zinc binding. These results suggested that HL can be used to monitor the changes of intracellular Zn2+ and should, therefore, be useful for clarifying the role of Zn2+ in biological processes in which the intracellular concentration of Zn2+ is important, for example, cell death induced by ischemia.45 Conclusion We have investigated the zinc ion fluorescence sensing and binding properties of 4-methyl-2,6-bis(((phenylmethyl)imino)methyl) phenol (HL), which displays high selectivity for Zn2+ and can be used as zinc ion-selective luminescent probe for biological application. The increase in emission in the presence of Zn2+ is accounted for by the formation of a metal-ligand complex (1). The X-ray crystal structure reveals that the 1 is a discrete hexanuclear complex, [Zn6(L)2(OH)2(CH3COO)8]. An approximately 6-fold Zn2+selective chelation-enhanced fluorescence response in HEPES (45) Koh, J.-Y.; Suh, S. W.; Gwag, B. J.; He, Y. Y.; Hsu, C.Y.; Choi, D. W. Science 1996, 272, 1013.

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buffer (pH 7.4) is attributed to the strong coordination of Zn(II) that would impose rigidity and hence decrease the nonradiative decay of the excited state. It has been demonstrated that this system in HEPES buffer is highly selective over other various metal ions. By the incubation of cultured living cells (B16F10 mouse melanoma and A375 human melanoma) with HL, intracellular Zn2+ concentration could be monitored. Acknowledgment. K.D. acknowledges the Council of Scientific and Industrial Research, New Delhi, India, for financial support. We appreciate the cooperation received from Professor Nitin Chattopadhyay, Department of Chemistry, Jadavpur University, Jadavpur, India. Supporting Information Available: Emission spectra of HL and HL with added base triethylamine, ESI-MS of 1, Job’s plot, time-resolved fluorescence decay, Table S1, ratiometric signaling of fluorescence, and details of the crystallographic study (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org. IC700420W